In examining a globe of the world, one can casually observe that it appears that the continents appear to “fit together” like pieces of a puzzle. This has lead to a number of theories in the development of the Earth with the geography as it appears today. In 1912, Alfred L. Wegener proposed a hypothesis of continental drift, suggesting that the Earth's continents had at one time been joined. In Wegener's hypothesis, one large continent existed that broke apart and land masses drifted through the ocean floor to where they are now located. Wegener was not the first to suggest that the continents had once been connected, but he was the first to present extensive evidence from several fields, including fossil evidence in support of his theories.
One model of continent formation and location is described in the theory of Plate Tectonics. This theory suggests that the surface of the Earth is composed of a number of plates which fit together like pieces of a spherical puzzle that are moving with respect to each other. The plate-tectonic model assumes a constant-sized Earth, and therefore postulates that sea-floor spreading that occurs at ridges in the Earth is compensated for by subduction and collisional compression.
In addition to Wegener and others' theories regarding continental movement, additional scientific camps have formed. One such group, called “Fixists,” believed that the continents and basins were essentially unchanged in their position and relative configuration. Another group, called “Contractionists,” believed that the solid Earth has gradually contracted over time, causing portions of the ocean floor to become dry land, and portions of dry land in turn to become ocean floor.
Another current theory, called the “expanding Earth” hypothesis, suggests that the shifting of the continents can be explained by a large increase in size of the Earth since its formation. As a spherical object expands radially, any object on its surface maintains its position and does not move horizontally on the surface. This can be envisioned by partially inflating a balloon, marking two points on the balloon, and more fully inflating the balloon. Inflating and deflating the balloon graphically illustrates how pieces of a once unitary land mass could separate. This expanding earth theory also has challenges, including questions regarding location and creation of mountains, and so forth.
A number of scientific developments have generated increased interest in various continent formation and location theories. Increased study of the ocean floor has demonstrated that the ocean floor is surprisingly rugged and contains material that is young relative to other areas of the Earth. This has further increased discussion of seafloor-spreading hypotheses, and theories regarding recycling of oceanic crust material. Studies of the patterns of the world's Earthquake and volcanic activity have revealed that such activity appears to be concentrated along oceanic trenches and submarine mountain ranges. Additionally, it appears that there have been repeated reversals of the Earth magnetic field over time. Specifically, stripes of magnetically different rock having alternating polarity have been located in rows on either side of the mid-ocean ridge. This phenomenon is suggested to be caused by the presence of grains of magnetite in the rock. The magnetite components of the rock behave like small magnets, and are aligned with the orientation of the Earth's magnetic field when the rock is in the magma phase. When the magma cools to form solid volcanic rock, the polarity of that portion of the rock is locked in, recording the Earth's magnetic orientation or polarity at the time of cooling. The oceanic crust therefore provides a record of the history of the reversals in the Earth's magnetic field, and therefore also the ages of different portions of the ocean floor.
Analysis tools to study the relationship of continents and other land masses on the Earth are always in demand to better help the scientific community recognize unique features that provide clues for solving the questions raised regarding continent formation and location.